The invention relates to a process for the winning of xylose by the hydrolysis of residues of annuals with acid solutions.
In the past several attempts have been made to obtain relatively pure xylose solutions so as to easily win pure xylose by crystallization.
Japanese Patent Application No. 55856/72 describes a process for the preparation of xylose from agricultural waste materials by hydrolysis with dilute sulfurous acid. The waste materials are first pre-washed with hot water or diluted acid. Then the pre-washed materials are hydrolyzed with a 0.05 to 2.0% aqueous solution of sulfur dioxide at a temperature of 130 to 170.degree. C. by which the xylans are converted to xylose. After concentration of the hydrolysate xylose is obtained by crystallization. The agricultural waste materials to be treated contain at least 20% by weight of xylans. By the pre-washing step with hot water or diluted acid the waste materials are subjected to 6 to 15% of weight loss on the basis of the dry substance. In the example pre-washing is carried out with water at 140.degree. C. for 90 minutes. Hydrolysis is carried out with 0.2% solution of sulfur dioxide at 160.degree. C. for 20 minutes, the pH of the mixture being 1.5. Xylose is won from the hydrolyzate in a manner known per se. Pre-washing with diluted acid is not illustrated. A critical discussion and a detailed description of the pre-washing step is lacking.
According to laid-open Dutch Patent Application No. 70,01592 the presence of impurities inhibiting the crystallization is substantially avoided by hydrolyzing comminuted shells of stone-fruits with mineral acids at an elevated temperature. This raw material is, however, not readily available in large quantities.
According to laid-open Dutch Patent Application No. 69,10072 vegetable materials such as grasses and wood are hydrolyzed with a diluted aqueous solution of oxalic acid at elevated temperatures. Preferably, the raw material is washed with boiling water before hydrolysis to remove free sugars, particularly glucose and galactose, and tanning and colouring substances. Since oxalic acid is expensive this known method is not economical.
French Patent Specification No. 1,477,305 describes the preparation of xylose and cellulose from vegetable materials containing cellulose by treating them with diluted mineral acids such as sulfuric acid, sulfurous acid or hydrochloric acid. The hydrolysis may be carried out either continuously or batchwise. This process yields a xylose solution which is heavily contaminated with organics and salts.
According to British Patent Specification No. 934,904 residues of tannin extraction and of sugar extraction are used as the raw material for the hydrolysis to xylose.
German Auslegeschrift No. 2,538,407 discloses washing with an aqueous solution of alkali before hydrolysis to remove acetic acid from the raw material. Since the hydrolysis is carried out with acids, washing with alkali necessitates washing with water and neutralization to remove alkali and salts before the hydrolysis is carried out.